Air pollution with exhaust gas from the internal combustion engines of automobiles and the like is not a new problem and it is now assuming the proportions of a social issue. Air pollution with exhaust gas is mainly attributable to unburnt hydrocarbons (hereafter called unburnt HC), carbon monoxide (hereafter called CO) and nitrogen oxides (hereinafter called NOx) contained in the exhaust gas. To render these contents of the gas harmless, various measures have been tried. To mention a few examples, there are purification systems utilizing an oxidizing catalyst, a reducing catalyst or a combination of these two. Also known is a system for simultaneous treatment of the three elements (three-way system) which can treat the unburnt HC, CO and NOx at the same time through reaction of them with a special catalyst under specific conditions. This three-way system is an excellent system and exhibits high purifying ability in the theoretical air/fuel ratio range. To maintain the theoretical air/fuel ratio range, the oxygen concentration of the exhaust gas in the exhaust system must be monitored so that the fuel jet volume in the engine can in turn be controlled. An oxygen sensor is used to monitor the oxygen concentration of the exhaust gas in the exhaust system.
The oxygen sensor takes the form of an oxygen concentration cell composed of a solid electrolyte which is made of a special ceramic material having oxygen-ion conductivity. The electromotive force or voltage generated through a difference in the oxygen partial pressure between the gas to be analyzed and a reference gas, is measured, and the oxygen concentration of the gas to be analyzed is determined from the measured voltage.
As the reference oxygen for the oxygen sensor, oxygen gas itself, or a metal oxide-metal can be employed. For the purpose of detecting the oxygen concentration in exhaust gas, it is convenient to use atmospheric air as the reference gas, i.e., the gas with the higher concentration of oxygen and the exhaust gas as the gas with the lower concentration of oxygen. In the oxygen sensor using atmospheric air as the reference gas, the air has to be admitted to the reference gas electrode and accordingly an air passage to introduce the air is provided in the oxygen sensor.
However, for the measurement of oxygen concentration in the exhaust gas, the oxygen sensor is installed in the engine room or compartment, etc., of an automobile where it is exposed to water, mud, etc., which can enter through the air passage and which could damage a heated sensor ceramic, or adversely affect the action of the electrodes.